The present invention relates generally to gas analysis, and more particularly to systems and methods for measuring concentrations of gases, including low-concentration or trace gases in the atmosphere.
The increasing concentrations of carbon dioxide and other trace gases in the atmosphere and the resulting greenhouse effect and climate change have become important topics for scientific research. In order to understand the global carbon balance, it is necessary to determine the rate at which carbon dioxide, other trace gases and energy exchanges occur between the atmosphere and terrestrial and oceanic ecosystems. A measurement technique called “eddy covariance” has been widely used to determine these rates. The air within a few hundred meters above the earth surface is mostly turbulent, so that turbulent structures (vortices of variable sizes) called “eddies” are responsible for the vertical transport of the most of the gases, including carbon dioxide, other trace gases, and water vapor, and also heat and momentum between the surface and the atmosphere. The rates of such transport can be calculated from simultaneous, high-frequency measurements of the vertical component of wind speed, the concentrations of carbon dioxide, or other trace gases, and water vapor and air temperature.
To measure concentrations of carbon dioxide, other trace gases, and water vapor, a gas analyzer can be used to analyze the transmittance of light in appropriate wavelength bands through a gas sample. With some gas analyzers, a sample gas containing unknown concentrations of a sampled gas and water vapor is placed in a sample cell, and a reference gas with zero or known concentrations of this gas and water vapor is placed in a reference cell. The analyzer measures the unknown gas concentrations in the sample cell from calibrated signals that are proportional to the difference between light transmitted through the sample cell and light transmitted through the reference cell. Other similar methods have also been used utilizing a non-absorbent optical filter and a chopper motor to emulate a zero (no absorbing) condition, and no reference cell then is required.
Presently, readily available laser technologies are generally not able to provide enough resolution required for sampling of low-concentration gases unless substantial averaging is applied to minimize errors and to achieve required specifications, or a significant pressure drop in the sample cell is used to spectroscopically enhance the absorption lines. Atmospheric trace gases of low concentrations (e.g., CH4, N2O, NH3, isotopes of CO2 and H2O, etc.). are therefore presently sampled in two major ways:                (I) slow sampling, when specifications are achieved by minimizing errors in measured concentrations by time-averaging; these may include flask, chamber, and mean concentration measurements; and        (II) fast sampling, when errors in measured concentrations are reduced by averaging out in a large sampling volume and/or over long optical sampling paths. In both cases a significant pressure drop in the sampling cell is typically utilized to spectroscopically enhance the absorption lines.The second, fast sampling approach is achieved in presently available instruments by either (II.a) using large sampling cell to allow the laser beam to pass through a large distance to be absorbed by a gas of interest, or (II.b) by the use of cavity ring down or related techniques that require very long optical paths (e,g, meters to kilometers) folded multiple times in a smaller high finesse cavity, which is highly sensitive to contamination, and can be impractical. The large sampling cell approach (II.a) presently requires very large flow in order to flush sample cell about 5-10 times per second (5-10 Hz) or more. Presently such flow is achieved by using a very powerful pump pulling air through the closed-path cell with a small intake opening (e.g., typically on the order of 0.5-1.0 cm) at 10-20× pressure drops. In both cases (II.a and II.b), the devices have high power demand, high maintenance requirements and unclear/uninvestigated consequences for measuring rapid gas fluctuations at 1/10 to 1/20 of the actual ambient air pressure.        
In addition, reactive gases (e.g., volatile organic compounds, etc.) and “sticky” gases, (e.g., ammonia) are very difficult to sample using traditional intake tubes as are present on most prior art devices as initial information is lost due to chemical reactions or smearing while traveling through the intake tube.
Therefore it is desirable to provide systems and methods that overcome the above and other problems.